Comprehensive Study Notes: The Respiratory System (Anatomy & Physiology)

The Respiratory System

Overview

The respiratory system is responsible for supplying the body with oxygen and removing carbon dioxide. It consists of both conducting zones (passageways for air) and respiratory zones (sites of gas exchange). Major structures include the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and lungs.

• Conducting Zone: Passageways for air to reach gas exchange sites; includes nose, nasal cavity, pharynx, larynx, trachea, and bronchi.

• Respiratory Zone: Site of pulmonary ventilation (breathing) and gas exchange; includes bronchioles, alveolar ducts, and alveoli.

Nose and Nasal Cavity

Functions of the Nose

• Provides airway for respiration

• Moistens and warms entering air

• Filters and cleans inspired air

• Serves as a resonating chamber for speech

• Houses olfactory receptors

Structural Divisions

• External Nose: Includes the root, bridge, dorsum nasi, and apex.

• Nasal Cavity: Divided by the nasal septum; opens posteriorly into the nasopharynx via internal nares; roof formed by ethmoid and sphenoid bones; floor formed by hard and soft palates.

Nasal Vestibule and Vibrissae

• Located just inside nostrils; contains sebaceous glands and hair follicles (vibrissae) that filter coarse particles.

Mucosal Linings

• Olfactory Mucosa: Contains olfactory receptors (superior region).

• Respiratory Mucosa: Pseudostratified ciliated columnar epithelium with goblet cells; secretes mucus and defensins (antimicrobial peptides).

• Cilia move contaminated mucus toward the throat for swallowing and digestion.

Nasal Conchae (Turbinates)

• Superior, middle, and inferior conchae project medially from the lateral walls.

• Increase mucosal surface area and enhance air turbulence to increase contact with mucosa.

• During inhalation, filter, heat, and moisten air; during exhalation, reclaim heat and moisture.

Paranasal Sinuses

• Located in frontal, sphenoid, ethmoid, and maxillary bones.

• Lighten skull and help warm and moisten air.

• Produce mucus that drains into nasal cavity.

Pharynx (Throat)

Structure and Regions

The pharynx is a muscular tube connecting the nasal cavity and mouth to the larynx and esophagus. It is divided into three regions:

1. Nasopharynx: Posterior to nasal cavity; serves only as an air passageway; contains pharyngeal tonsil and openings of auditory tubes.

2. Oropharynx: Posterior to oral cavity; passageway for food and air; contains palatine and lingual tonsils; lined with stratified squamous epithelium for protection.

3. Laryngopharynx: Posterior to epiglottis; extends to larynx; serves as a common pathway for food and air; lined with stratified squamous epithelium.

Larynx (Voice Box)

Functions

• Provides an open airway

• Routes food and air into proper channels

• Responsible for voice production

Structure

• Extends from hyoid bone to trachea

• Composed of nine cartilages connected by membranes and ligaments

Swallowing Mechanism

• Larynx rises, epiglottis moves downward to cover laryngeal inlet, preventing food from entering airway and directs it to the esophagus

Voice Production

• Vocal ligaments form vocal cords

• Glottis: Opening between vocal cords

• Pitch determined by length and tension of vocal cords

• Sound shaped by pharynx, tongue, soft palate, and lips

Trachea (Windpipe)

• Extends from larynx into mediastinum

• Composed of C-shaped cartilage rings and ciliated epithelium

• Divides into right and left main bronchi

Bronchi and Bronchioles

• Right bronchus: wider, shorter, more vertical

• Each main bronchus divides into secondary (lobar) bronchi, then tertiary (segmental) bronchi

• Bronchioles: less than 1 mm diameter; terminal bronchioles lead to respiratory bronchioles

• Epithelium transitions from pseudostratified to cuboidal

Respiratory Zone Structures

• Terminal bronchioles lead to respiratory bronchioles, which open into alveolar ducts, alveolar sacs, and alveoli

• Approximately 300 million alveoli provide large surface area for gas exchange

Respiratory Membrane

• Alveolar and capillary walls plus fused basement membranes

• Allows rapid diffusion of gases

Cell Types in Alveoli

• Type I cells: Simple squamous epithelium

• Type II cells: Secrete surfactant and antimicrobial proteins

• Alveolar macrophages: Remove debris and pathogens

Lungs

• Occupy most of thoracic cavity

• Right lung: 3 lobes; left lung: 2 lobes

• Each lobe divided into bronchopulmonary segments

Bronchopulmonary Segments

• About 10 segments per lung, each served by its own artery and vein

• Segments can be surgically removed without affecting adjacent tissue

Pulmonary Circulation

• Pulmonary arteries: Carry deoxygenated blood to lungs for gas exchange

• Pulmonary veins: Return oxygenated blood to heart

Pleurae

• Double-layered serous membrane surrounding the lungs

• Parietal pleura: Lines thoracic wall, diaphragm, mediastinum

• Visceral pleura: Covers external lung surface

• Pleural fluid fills pleural cavity, reducing friction during breathing

Pulmonary Ventilation

Composition of Air

• Mixture of gases: Nitrogen, Oxygen, Carbon Dioxide, Water Vapor, and trace gases

• Partial pressure of oxygen (PO2) = 160 mmHg

Pressure Relationships

• Atmospheric Pressure (Patm): Pressure exerted by air surrounding the body; at sea level, 760 mmHg

• Intrapulmonary Pressure (Ppul): Pressure within alveoli; fluctuates with breathing

• Intrapleural Pressure (Pip): Pressure within pleural cavity; always negative relative to Ppul

• Transpulmonary Pressure: Difference between Ppul and Pip; keeps lungs inflated

Mechanics of Breathing

• Inspiration: Active process; diaphragm and external intercostal muscles contract

• Expiration: Normally passive; muscles relax, thoracic cavity volume decreases

Boyle's Law: Describes the relationship between pressure and volume of a gas:

$P_1 V_1 = P_2 V_2$

As volume increases, pressure decreases, and vice versa.

Physical Factors Influencing Pulmonary Ventilation

• Airway Resistance: Friction in airways; usually insignificant in healthy individuals

• Alveolar Surface Tension: Surfactant reduces surface tension, preventing alveolar collapse

• Lung Compliance: Measure of lung stretchability; high compliance = easy expansion

Respiratory Volumes and Capacities

Respiratory Volumes

• Tidal Volume (TV): Amount of air inhaled or exhaled in one breath (~500 mL)

• Inspiratory Reserve Volume (IRV): Amount of air that can be inhaled after normal inspiration

• Expiratory Reserve Volume (ERV): Amount of air that can be exhaled after normal expiration

• Residual Volume (RV): Air remaining in lungs after forced expiration

Respiratory Capacities

• Inspiratory Capacity (IC): Total amount of air that can be inspired after normal tidal expiration (IC = TV + IRV)

• Functional Residual Capacity (FRC): Volume of air remaining in lungs after normal tidal expiration (FRC = ERV + RV)

• Vital Capacity (VC): Total amount of exchangeable air (VC = TV + IRV + ERV)

• Total Lung Capacity (TLC): Sum of all lung volumes (TLC = TV + IRV + ERV + RV)

Dead Space

• Anatomical Dead Space: Air in conducting zone structures (~150 mL)

• Alveolar Dead Space: Nonfunctional alveoli due to collapse or obstruction

• Total Dead Space: Sum of anatomical and alveolar dead spaces

Physical Properties of Gases

Dalton's Law of Partial Pressures

• Total pressure exerted by a mixture of gases is the sum of the partial pressures of each gas

• Each gas's partial pressure is directly proportional to its percentage in the mixture

Henry's Law: When a gas is in contact with a liquid, it dissolves in proportion to its partial pressure.

Pulmonary Gas Exchange

External Respiration

Exchange of O2 and CO2 across the respiratory membrane between alveoli and pulmonary capillaries.

• Driven by partial pressure gradients and gas solubilities

• Influenced by thickness and surface area of respiratory membrane

Internal Respiration

Exchange of gases between systemic capillaries and body tissues.

Transport of Molecular Oxygen and Carbon Dioxide

Oxygen Transport

• Most O2 is transported bound to hemoglobin in red blood cells

• Small amount dissolved in plasma

Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen

Reduced Hemoglobin (HHb): Hemoglobin that has released oxygen

Mechanism of Oxygen Transport

1. O2 loading occurs in pulmonary capillaries

2. O2 unloading occurs in systemic capillaries

Hemoglobin and Oxygen Affinity

• Affinity for O2 increases as each O2 molecule binds

• Factors affecting affinity: PO2, temperature, pH, PCO2, BPG

Carbon Dioxide Transport

• Transported in three forms: dissolved in plasma, bound to hemoglobin, as bicarbonate ions (HCO3-)

Carbonic Anhydrase: Enzyme in RBCs that catalyzes conversion of CO2 and H2O to carbonic acid, which dissociates to bicarbonate and H+.

$\mathrm{CO_2 + H_2O \xrightarrow{carbonic\ anhydrase} H_2CO_3 \rightarrow HCO_3^- + H^+}$

Bohr and Haldane Effects

• Bohr Effect: Increased CO2 or H+ decreases Hb-O2 affinity, enhancing O2 unloading

• Haldane Effect: Reduced hemoglobin increases CO2 loading

Carbonic Acid–Bicarbonate Buffer System

• Bicarbonate ions act as the alkaline reserve of the blood's buffering system

• Helps maintain acid-base balance

Control of Respiration

Medullary Respiratory Centers

• Dorsal Respiratory Group (DRG): Integrates input from peripheral stretch and chemoreceptors

• Ventral Respiratory Group (VRG): Sets basic rhythm of breathing

Pontine Respiratory Centers (Pons)

• Influence and modify activity of medullary centers

• Smooth out transitions between inspiration and expiration

Rate and Depth of Breathing

• Determined by how actively respiratory centers stimulate respiratory muscles

• Influenced by chemical and neural changes

Chemical Factors Influencing Respiration

• CO2: Most potent stimulus; increased CO2 increases rate and depth of breathing

• O2: Decreased O2 stimulates peripheral chemoreceptors

• pH: Decreased pH (from lactic acid or ketone bodies) stimulates increased ventilation

Higher Brain Center Influence

• Hypothalamic and Limbic System Control: Modify respiratory rate and depth in response to emotion and temperature

• Voluntary Control (Cerebral Cortex): Conscious control over breathing, such as during speech or holding breath

Respiratory Volume	Definition	Typical Value (mL)
Tidal Volume (TV)	Amount of air inhaled/exhaled in one breath	~500
Inspiratory Reserve Volume (IRV)	Air that can be inhaled after normal inspiration	~3100
Expiratory Reserve Volume (ERV)	Air that can be exhaled after normal expiration	~1200
Residual Volume (RV)	Air remaining in lungs after forced expiration	~1200

Respiratory Capacity	Formula	Definition
Inspiratory Capacity (IC)	TV + IRV	Total air that can be inspired after normal expiration
Functional Residual Capacity (FRC)	ERV + RV	Air remaining in lungs after normal expiration
Vital Capacity (VC)	TV + IRV + ERV	Total exchangeable air
Total Lung Capacity (TLC)	TV + IRV + ERV + RV	Sum of all lung volumes

Example: During exercise, increased CO2 production stimulates chemoreceptors, increasing respiratory rate and depth to maintain homeostasis.

Additional info: These notes expand on the original content by providing definitions, formulas, and examples for key concepts in respiratory anatomy and physiology, suitable for college-level exam preparation.